1. Field of the Invention
The present invention relates to an image pickup apparatus and more particularly to an image pickup apparatus with a function of manually moving a lens.
2. Related Background Art
A conventional lens exchangeable system of an image pickup apparatus such as a video camera will be described with reference to the block diagram shown in FIG. 1. In a conventional lens unit whose magnification can be changed, a magnification lens 802 and a correction lens 803 are mechanically coupled by a cam. Upon manual or electrical operation of variable magnification, the magnification lens 802 and correction lens 803 move in unison.
A combination of the magnification lens 802 and correction lens 803 is generally called a zoom lens. In such a lens system, a front lens 801 is used as a focus lens and a focus is adjusted by moving it in the optical axis direction. Light passed through this lens group is focussed on an image plane of an image pickup element 804, and photoelectrically converted into electric signals which are output as video signals.
This video signal is sampled and held by a CDS/AGC 805, thereafter amplified to a predetermined level, and converted into a digital video signal by an A/D converter 806. Thereafter, this digital video signal is input to a processing circuit of a camera for converting it into a standard television signal, and also to a band-pass filter (BPF) 807.
The high frequency components of the video signal are derived by BPF 807, and signals corresponding only to a focus detection area in an image frame are picked up by a gate circuit 808. The peak of this picked-up signal is held by a peak-hold circuit 809 at an interval synchronous with an integer multiple of the vertical (V) sync signal to thereby generate an AF evaluation value.
This AF evaluation value is supplied to a main body AF microcomputer 810 in the camera main body. This AF microcomputer 810 determines a focussing speed corresponding to an in-focus state a motor drive direction increasing the AF evaluation value, and transmits the speed and direction of a focus motor to a lens microcomputer 811.
The lens microcomputer 811 controls the speed and direction of a motor 813 via a motor driver 812 in accordance with instructions from the main body microcomputer 810 so as to move the focus lens 801 in the optical axis direction and adjust the focus.
In response to the operation of a zoom switch 818, the main body microcomputer 810 supplies the drive speed and direction of the zoom lenses 802 and 803 to a zoom motor driver 814 in the lens unit 816 to control a zoom motor 815 and drive the zoom lenses 802 and 803 to thereby conduct zooming. The camera main body 817 is so constructed that the lens unit 816 is dismounted therefrom and it is replaced by another lens unit to allow a user to take variety of photograph scenes.
In the case of integral type cameras of public use, the magnification lens 802 and correction lens 803 are not mechanically coupled by a cam but a motion locus of the correction lens is stored in the microcomputer as lens cam data, in order to make the camera compact and allow a user to take a scene just in front of the lens.
The correction lens 803 is driven in accordance with the stored lens cam data and is constructed so as to perform focussing by itself. This lens system so called an inner focus type is becoming the main trend, and has advantages of low cost, system simplicity, and small and light lens barrel.
FIG. 2 briefly shows the structure of a conventional inner focus type lens system. In FIG. 2, reference numeral 901 represents a first fixed lens group, reference numeral 902 represents a second magnification lens group, reference numeral 903 represents an iris, reference numeral 904 represents a third fixed lens group, reference numeral 905 represents a fourth lens group (hereinafter called a focus lens) with a focussing function and a function called a compensation function of compensating for a change in the focus plane position caused by magnification, and reference numeral 906 represents an image plane of an image pickup element.
In the lens system constructed as shown in FIG. 2, the fourth lens group 905 has both the compensation function and the focussing function. Therefore, even if the focal length is the same, the position of the fourth lens group 905 for the control of focussing to the image plane 906 changes with the subject distance.
The graph shown in FIG. 3 shows the positions of the fourth lens group 905 for focussing to the image plane 906, the positions being continuously plotted as the subject distance is changed at each focal length. The fourth lens group 905 moves during magnification on one of loci shown in FIG. 3 identified by the subject distance so that zooming without unsharp focus is possible.
In the lens system of a front lens focus type, the compensation lens is provided independently from the magnification lens, and the magnification lens and compensation lens are mechanically coupled by a cam ring.
For example, if a manual zoom knob is mounted on this cam ring and even if it is moved fast to manually change the focal length, the cam ring rotates following the motion of the knob and the magnification lens and compensation lens move along the cam groove of the cam ring. Therefore, if the focus lens is set just in-focus, the above operation will not make unsharp focus.
In contrast, for the general control of the inner focus type lens system, information of a plurality of locus shown in FIG. 3 is stored in a certain form (loci themselves or a function of loci using a lens position as a variable), and in accordance with the positions of the focus lens and magnification lens, the locus is selected along which zooming is performed.
Further, the position of the focus lens relative to the position of the variable lens is read from a memory and used for the control of lenses. It is therefore necessary to read the position of each lens at some degree of precision. As seen from FIG. 3, if the magnification lens moves at a speed, particularly at a constant speed or near it, the slope of the locus of the focus lens changes from time to time as the focal length changes.
This means that the speed and motion of the focus lens changes from time to time. In other words, an actuator for the focus lens is required to have a precise speed response in the range from 1 Hz to several hundreds Hz.
As an actuator meeting such requirements, stepping motors are generally used for the focus lens group of the inner focus lens system. Each stepping motor rotates in complete synchronization with stepping pulses supplied from a lens control microcomputer or the like. Since the stepping angle per one pulse is constant, it is possible to realize high speed response characteristics, high stop precision and high position precision.
Since the rotary angle of a stepping motor is proportional to the number of stepping pulses, these pulses themselves can be used for an increment type encoder without using an additional position encoder.
As described above, in order to perform magnification while maintaining in-focus state by using a stepping motor, it is necessary to store locus information shown in FIG. 3 in a microcomputer or the like in some form (loci themselves or a function of loci using a lens position as a variable), to read the locus information in accordance with the position of the magnification lens or its motion speed, and to move the focus lens in accordance with the read information.
FIG. 4 is a graph illustrating an example of a conventional locus tracing method. In FIG. 4, Z0, Z1, Z2, . . . , Z6 represent the positions of the magnification lens, a0, a1, a2, . . . , a6 and b0, b1, b2, . . . , b6 represent the typical locus positions stored in a microcomputer, and p0, p1, p2, . . . , p6 represent the locus positions calculated by the above two sets of locus positions.
The calculation equation of the loci is given by                                                                         p                ⁡                                  (                                      n                    +                    1                                    )                                            =                            ⁢                                                                                                                                      p                        ⁡                                                  (                          n                          )                                                                    -                                              a                        ⁡                                                  (                          n                          )                                                                                                                          /                                                                                                        b                        ⁡                                                  (                          n                          )                                                                    -                                              a                        ⁡                                                  (                          n                          )                                                                                                                                        ×                                                                                                      ⁢                                                                                                            b                      ⁡                                              (                                                  n                          +                          1                                                )                                                              -                                          a                      ⁡                                              (                                                  n                          +                          1                                                )                                                              -                                          a                      ⁡                                              (                                                  n                          +                          1                                                )                                                                                                              +                                  a                  ⁡                                      (                                          n                      +                      1                                        )                                                                                                          (        1        )            
According to this equation (1), if the focus lens is at the position, for example, p0 in FIG. 4, an interior division ratio of p0 to a line segment b0-a0 is calculated and a point p1 as an interior division of a line segment b1-a1 is calculated by using the calculated interior division ratio. The motion speed of the focus lens for maintaining an in-focus state can be calculated from the position difference p1-p0 and the time required for the magnification lens to move from Z0 to Z1.
Next, it is assumed that there is no such a limit as the stop position of the magnification lens is only on the boundary represented by the stored typical locus data. FIG. 5 is a graph illustrating an interpolation method for calculating the position of the magnification lens, this graph showing part of the graph of FIG. 4 and illustrating the calculation of an optional intermediate position of the magnification lens.
In FIG. 5, the ordinate represents the focus lens position and the abscissa represents the zoom lens position. The typical locus positions (focus lens positions relative to the magnification lens) stored in the lens control microcomputer are indicated at the magnification lens positions Z0, Z1, . . . , Zk−1, Zk, . . . , Zn. The corresponding positions of the focus lens for respective subject distances are indicated as:
a0, a1, . . . , ak−1, ak, . . . , an, and
b0, b1, . . . , bk−1, bk, . . . , bn.
Assuming that the magnification lens position is at Zx not on the zoom boundary corresponding to the stored typical locus position and the focus lens position is px, then ax and bx are given by:ax=ak−(zk−zx)×(ak−ak−1)/(Zk−Zk−1)  (2)bx=bk−(Zk−Zx)×(bk−bk−1)/(Zk−Zk−1)  (3)
The positions ax and bx can therefore be calculated by interiorly dividing pairs of the stored four typical locus position data (ak, ak−1, bk, bk−1) at the same subject distance by the interior division ratio obtained by the present magnification lens position and two zoom boundary positions (e.g., Zk and Zk−1 shown in FIG. 5) on both sides of the present magnification lens position.
The positions pk and pk−1 can be calculated by interiorly dividing pairs of the stored four typical locus position data (ak, ak−1, bk, bk−1) at the same subject distance by the interior division ratio obtained from ax, px and bx as in the equation (1).
For the zooming in the direction from a wide end to a telephoto end, the motion speed of the focus lens for maintaining an in-focus state can be calculated from the position difference between the target focus position pk and the present focus position px and the time required for the magnification lens to move from Zx to Zk. For the zooming in the direction from the telephoto end to the wide end, the motion speed of the focus lens for maintaining an in-focus state can be calculated from the position difference between the target focus position pk−1 and the present focus position px and the time required for the magnification lens to move from Zx to Zk−1. The above locus tracing method has been proposed heretofore.
As described earlier, in the inner focus type lens unit, a stepping motor is used as an actuator to make the driver system compact and simple. Further, stepping pulses of the stepping motor can be generated easily in the lens control microcomputer. Therefore, by counting the number of stepping pulses output from the lens control microcomputer, the position of a lens can be known precisely without using an additional encoder or the like for lens position detection.
In a front lens focus type lens system, a general zooming mechanism of moving a zoom lens mechanically connected to a zoom sleeve by rotating the zoom sleeve fitted in a lens barrel, is excellent in the following points and other points.
(1) The lens moves proportional to a rotation amount.
(2) Therefore, zooming can be performed smoothly in the range from coarse to fine adjustment.
However, in the inner focus type lens system, it is difficult to mechanically couple a lens to a zoom sleeve and move it with an external force and to attain manual zooming, from the following reasons and others.
(1) Movable lenses are all mounted in a lens barrel.
(2) If a lens is rotated by a cam ring or the like mechanically coupled thereto without using a specific control circuit, a difference may be generated between the count of drive pulses of a stepping motor and the actual lens position.
(3) The drive system of a simple structure is not suitable for manual operation.
If a zoom handling member is not provided on the lens side of a camera, particularly a lens exchangeable camera shown in FIG. 1, a user must hold the camera with its lens barrel although depending upon the type of mounted lens. Therefore, a user is required to temporarily stop viewing the finder and find a zooming operation switch on the main body for the view angle adjustment. In such a case, a camera shake may occur or smooth photographing may be hindered.
To solve the above problems, another system has been proposed in which an encoder is fitted in a lens barrel and the zoom lens is moved by electrically detecting the direction and speed of the encoder. In this specification, a zoom sleeve not mechanically coupled to the zoom lens is called a “zoom ring”. The structure of a zoom ring will be described with reference to FIGS. 12, 13, 14A and 14B.
In FIG. 12, reference numeral 112 represents a rotary type encoder to be fitted in a lens barrel, and reference numeral 112a represents a comb structure of the encoder constituted of a light transmitting portion and a light reflecting portion. Reference numerals 113a and 113b each represent a photodetector (sensor) comprising a light projector 120 and a light detector 121. An output signal of the photodetector 113a changes its state between when it receives light reflected from the comb structure 112a and when it does not receive the reflected light (FIG. 13 is an enlarged view of a portion 122 surrounded by a broken line in FIG. 12).
As the encoder 112 rotates, the output signals generated by the photodetectors 113a and 113b change as shown in FIG. 14A or 14B. The positional relationship between the photodetectors 113a and 113b is determined so that the phases of two output signals shift by a proper amount. The rotation speed is detected from a period of the output signal, and the rotation direction is detected from a phase relationship between two signals.
Specifically, the output waveforms shown in FIG. 14A stand for the normal rotation of the encoder, and the output waveforms shown in FIG. 14B stand for the reverse rotation of the encoder. By picking up the output signals from a combination of the photodetectors 113a and 113b, the drive direction and speed of the lens are determined from the picked-up signals.
As the zoom ring with the encoder illustrated in FIGS. 12, 13, 14A and 14B rotates, a lens actuator such as a stepping motor is driven. Although this lens system is of an inner focus type, the operation performance thereof is similar to a front lens focus type lens system and zooming can be performed smoothly through power zooming.
If a video camera of general use having the above-described manual zooming mechanism is held with the left hand of a user and the zoom sleeve is rotated with the right hand, the zoom lens is rotated in a clockwise direction toward the telephoto end with the user's arm being opened, and rotated in the counter-clock wise direction toward the wide end with the arm being closed, in order to make it easy to use the wide/macro function of the front lens focus type.
In contrast, in the case of video cameras for business use, the zoom lens is rotated in the counter-clockwise direction with the arm being closed for the telephoto end in order to suppress camera shake. As above, the motion direction of the zoom lens relative to the rotation direction of the manual zooming operation member has been mechanically or electrically fixed depending upon the use application of cameras.
Therefore, depending upon photographing purpose and upon which hand is used, the camera operation and photographing may become difficult. If a user wants a specific camera easy to use, the camera must be ordered as a custom made and becomes very expensive.
A camera, particularly a camera of a lens exchangeable system capable of using an ultra-wide or ultra-telephoto lens, may often be used both for general and business uses. It is therefore undesirable to fix the motion direction of the zoom lens relative to the rotation direction of the zoom sleeve.